The present invention relates to a method of producing a sintered body comprising mixing one or more powders forming hard constituents and powder forming binder phase comprising cobalt, wherein the cobalt powder mainly has a face centered cubic (fcc) structure. The present invention also relates to a granulated “ready-to-press” powder comprising one or more hard constituents, organic binders and powders forming binder phase comprising cobalt, wherein the cobalt powder mainly has a face centered cubic (fcc) structure. The present invention also relates to a sintered body made according to the method of the invention.
Sintered bodies like round tools, cutting tool inserts etc. are usually made from materials containing cemented carbides or titanium based carbonitride alloys, often referred to as cermets. These materials contain one or more hard constituents such as carbides or carbonitrides of e.g. tungsten, titanium, tantalum, niobium, chromium etc together with a binder phase. Depending on composition and grain size, a wide range of materials combining hardness and toughness can be used in many applications, for instance in rock drilling and metal cutting tools, in wear parts etc. The sintered bodies are made by techniques common in powder metallurgy like milling, granulation, compaction and sintering.
The use of cobalt as a binder phase when manufacturing cemented carbides and cermets is well known in the art.
Cobalt is allotropic, that is, at temperatures less than about 417° C., pure cobalt atoms are arranged in a hexagonal close packed (hcp) structure and at temperatures more than about 417° C., pure cobalt atoms are arranged in a face centered cubic (fcc) structure. Thus, above 417° C., pure cobalt exhibits an allotropic transformation, i.e. the hcp-structure changes to fcc-structure.
The cobalt powder conventionally used when manufacturing sintered bodies such as drills, cutting tool inserts etc. usually has an hcp-structure. However, in a sintered body the cobalt binder phase has an fcc-structure which is obtained during the sintering operation.
During manufacturing of sintered bodies it is important that the cobalt powder is easily dispersed during milling or mixing. This is especially important when making sintered bodies of fine grain materials, materials with low amounts of binder or by using raw materials whose properties may be destroyed by intense milling. Fine grained raw materials usually require higher compaction pressures which normally are not desired due to the risk of pressing cracks in the pressed bodies, abnormal wear and even risk of compaction tool failure. Due to this, a decrease in compaction pressure is desired.
Several attempts have been made to improve the quality of the cobalt powder to make it more dispersible. Cobalt with smaller grains, down to 0.5 μm, has been produced industrially and also, a transition from an elongated to a spherical morphology has been done. Different techniques have also been developed to coat the hard constituents to obtain a composite powder with well distributed cobalt without milling.
EP 0578720 A discloses a method of making cemented carbide articles using binder phase powders with spherical, non-agglomerated particles. The use of such binder powders, preferably cobalt powders, gives sintered bodies with reduced porosity.
WO 98/03691 discloses a method of making cemented carbide with a narrow grain size distribution. To obtain a material with narrow grain size distribution the tungsten carbide is coated with cobalt prior mixing with other constituents. Further, the mixing method is chosen so that no change in grain size or grain size distribution occurs.
However, further improvements regarding cracks, porosity, dispersibility of the cobalt etc. are still required. The present invention disclosed herein further improves properties like dispersibility, pressing cracks and porosity.